A process for producing plexifilamentary strands of fiber-forming polymers is described in U.S. Pat. No. 3,081,519 to Blades et al. Specifically, Blades et al. disclose extruding a homogeneous solution of a fiber-forming polymer in a liquid, which is a non-solvent for the polymer below its normal boiling point, at a temperature above its normal boiling point, into a medium of lower temperature and pressure. The vaporizing liquid within the extrudate forms bubbles, breaks through the confining walls and cools the extrudate, causing solid polymer to form therefrom. The resulting multifibrous yarn-like strand has an internal fine structure or morphology which may be characterized as a three-dimensional integral plexus consisting of a multitude of essentially longitudinally extended interconnecting random length fibrous elements often referred to as film fibrils. A strand comprising a three-dimensional network of film-fibril elements is generally referred to as a plexifilament.
In U.S. Pat. No. 3,227,794 to Anderson et al., a process is disclosed for flash spinning a fibrillated plexifilamentary material by supplying polymer and solvent for the polymer at elevated temperature and pressure in order to provide a solution containing 2 to 20 wt. % polymer. Thereafter, the solution is passed to a zone having a pressure above the two-liquid-phase pressure boundary for the solution and discharged through an extrusion orifice of restricted size into an area of substantially atmospheric temperature and pressure. As a result, the solvent almost instantaneously evaporates and cools the polymeric material during adiabatic expansion to form a solidified plexifilamentary strand.
The plexifilamentary strands produced by these processes are particularly useful in preparing nonwoven fibrous sheets as described in U.S. Pat. No. 3,169,899 to Steuber. In preparing such sheets, the extruded material is passed horizontally from the extrusion orifice directly to the surface of a rotating or oscillating deflector which opens the plexifilament into a wide web. The deflector directs the web downward onto a moving collection belt so that it is collected in random, multidirectional, overlapping layers. In order to electrostatically pin the web to the belt, means are provided for creating opposite charges on the strands and the collection belt. The flash spinning apparatus, comprising extrusion orifice, electrostatic charging means and deflector means for directing plexifilamentary strands onto the collection belt, is typically referred to as a spin pack.
The solvents used in these processes are generally haloalkanes and have normal boiling points at least about 25.degree. C. below the polymer's melting temperature. Trichlorofluoromethane (CFC-11) and methylene chloride are frequently employed at levels of from about 80 to about 90% by weight of the polymer solution. Since these large quantities of solvent form no part of the ultimate sheet product, it is both economically and environmentally desirable that the solvent be reclaimed and reused.
Typically, the solvent is reclaimed by extruding the solution in a substantially closed spin-cell which surrounds the spin packs and other sheet-forming equipment. U.S. Pat. No. 3,504,076 to Lee discloses improving the properties of nonwoven plexifilamentary webs of polyethylene by utilizing a substantially closed spin-cell in which the gaseous atmosphere is maintained at temperatures of from about 34.degree. C. to 60.degree. C. The gaseous atmosphere inside the spin-cell is predominantly solvent vapor with a minor volume of overhead air that covers the solvent vapor. The solvent is primarily recovered by withdrawing, compressing and then condensing the solvent vapor. The atmosphere surrounding the spin packs is preferably maintained at 100% solvent vapor so that the overhead air does not affect electrostatic charging of the web. Therefore, the overhead air covering the solvent vapor must be continously maintained above the level of the spin packs in order for the electrostatic charging and laydown process to work effectively. In addition, the solvent vapor level should not rise above the ceiling of the spin-cell since solvent vapor lost during spin pack maintenance represents a potential hazard to stratospheric ozone.
Clearly, what is needed is a method for controlling the level of vapor solvent contained in a vessel, and more particularly a spin-cell. It is therefore an object of the present invention to provide a method for controlling the level of the solvent vapor/overhead gas interface contained in a spin-cell used to produce nonwoven film-fibril sheets. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description of the invention which hereinafter follows.